A water pipe burst and caused the collapse of underground shaft in a gold mine near Johannesburg, where about 3,000 workers got trapped.
It was not immediately possible to get comment from the management of Harmony Gold's Elandsrand Mine in Carletonville, nor details, such about how deep down the shaft is.
The spokesman for the National Union of Mineworkers, Lesiba Seshoka, said the managers were meeting with union members.
Miners had gone down the shaft, then the water pipe burst, and they had not been heard from since about 10 a.m. (0900 GMT), he said.
"It's a terrible situation," Seshoka told The Associated Press. "The only exit is blocked, probably by a fall of ground."
Gold mine shafts in South Africa are typically 3,000 meters below ground or more, he said.
The union feared the men could be trapped without oxygen because of collapsed ground, or impeded by rock falls and mud slides by the burst ware pipe.
Seshoka charged the shafts had not been properly maintained. "Our guys there tell us that they have raised concerns about the whole issue of maintenance of shafts with the mine (managers) but they have not been attended to," he said.
Mine managers and owners of the South African-owned business could not be reached for comment late Wednesday night.
Last year, 199 mineworkers died in accidents, mostly rock falls, the government Mine Health and Safety Council reported in September.